1. Field of Invention
The present invention relates to a device such as a timepiece that has a housing and a magnetically shielded component that is magnetically shielded from an external magnetic field, and to a method of manufacturing a housing material used as an external component of the device.
2. Description of Related Art
Stepping motors that have an electromagnet made from a drive coil and a stator, a rotor made from a permanent magnet, and use the repulsion of the electromagnet and rotor to drive the rotor are commonly used in timepiece movements to drive the hands in modern electronic timepieces. Such movements may cease operating normally when exposed to a strong external magnetic field. The movement is therefore covered with a dedicated magnetic shield. A structure with this dedicated magnetic shield according to the related art is shown in FIG. 2, but the structure is described in detail in contrast to a preferred embodiment of the invention below.
Components used in the case member of a timepiece must meet certain standards in terms of hardness, corrosion resistance, appearance, and other surface properties. The case member components of a timepiece must also be easily manufactured or processed to the desired shape and size in order to achieve the desired timepiece design or size.
Japanese Unexamined Patent Appl. Pub. JP-A-H09-31505 teaches a method of manufacturing a timepiece case member by mixing and kneading an organic binder with an alloy powder of ferritic stainless steel, extrusion molding, degreasing, and then sintering the component. The timepiece case member components resulting from this manufacturing method can be used as a magnetic shield, and are easily worked.
However, because a ferritic material is used, the resulting components do not offer the hardness, corrosion resistance, appearance, and other surface properties that are most important in a timepiece case member. More specifically, this material is not practical for use as a case member.
Austenitic stainless steel, however, offers excellent surface properties including strength and corrosion resistance, and is therefore used in case members to take advantage of these properties. However, because austenitic stainless steel is difficult to process, processing austenitic stainless steel to the desired shape is difficult and manufacturing costs therefore rise.
Japanese Unexamined Patent Appl. Pub. JP-A-2004-68115 and Japanese Unexamined Patent Appl. Pub. JP-A-2006-316338 therefore teach methods of first using easily processed ferritic stainless steel to produce case member components with the desired shape, and then austenitize the surface using nitrogen to achieve the strength, corrosion resistance, and other surface properties desired in a case member.
However, these methods simply teach technology for using ferritic stainless steel to produce case member parts of the desired shape from ferritic stainless steel, and then austenitize the surface using nitrogen gas in order to achieve the strength, corrosion resistance, and other surface properties desired in a case member, and are silent regarding the internal structure of the ferrite phase after the austenitizing process.
The austenitizing process using nitrogen gas enables the nitrogen atoms to penetrate the ferrite phase from both front and back sides of the work piece, and the portion in which the nitrogen concentration rises to at least a predetermined level changes to the austenite phase. The rate at which the nitrogen penetrates the ferrite phase is not uniform and varies according to the location. As a result, the nitrogen concentration is also not uniform, and the interface between the austenitized parts of the surface layer and the ferritic phase of the internal layer left below is particularly uneven. Depending on the degree of austenitizing, the austenitized surface layers on the opposite sides of the work piece may meet internally. In this case a uniform ferrite layer cannot be maintained and the internal ferrite phase layer may not be contiguous.
While separation of the internal ferrite phase layer does not affect the surface properties of the case member component, it does affect performance as a magnetic shield. More simply, the resulting part is unsuitable for use as a magnetic shield.